Chemical composition and process for aluminum etching



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United States Patent CHEMICAL COMPOSITION AND PROCESS FOR ALUMINUM ETCHING Robert Houston Elliott, Jr.,'0reland, Pa., assignor to Pennsalt Chemicals Corporation, a corporation of Pennsylvania This invention relates to the etching of aluminum and aluminum base alloys and more particularly to the rapid etching of aluminum and aluminum base alloys and compositions for obtaining such rapid etch.

The term etching as used herein, both in the specification and claims, includes not only light surface etching, but chemical dissolution of substantial amounts of the metal treated as occurs when forming metal objects through controlled selective chemical dissolution, a process frequently referred to as chem milling.

It is frequently desirable in the fabrication of construction parts from aluminum, for example in the fabrication of airplane parts, to produce parts which are relatively light in weight and which still retain relatively high mechanical strength. In order to reduce the Weight of such parts without impairing their strength, it is the practice to remove as much of the metal as possible from the portions of the part which do not contribute to its mechanical strength. It is sometimes extremely difficult, if not impossible, to accomplish this by the conventional methods of mechanical milling or machining for the reasons that the parts may be highly complex in shape. Also, by mechanical removal of the excess metal the strength of the part may be adversely affected. For these reasons, the practice has recently developed of removing this excess metal by chemical means.

It is known that construction parts made from aluminum and aluminum base alloys can be subjected to an alkaline etching bath, under controlled conditions of concentration and temperature, whereby metal, not protected, is dissolved. The portions of the part from which it is not desired to remove metal are protected by some suitable masking material which usually consists of a coating of chlorinated neoprene, polyvinyl chloride or latex-base film, or other alkali-resistant coating. In addition, delicately fretted or perforated articles are frequently manufactured by mechanically working two metals, only one of which is soluble in a particular etching bath, into the desired shape and then removing the soluble metal in the etching bath. By these methods, highly complex shapes are more easily formed.

It has been discovered that the etching rate of alkaline etching baths for aluminum and aluminum base alloys can be substantially increased by the addition of small amounts of one or more of the metals of the group consisting of molybdenum, iron and nickel. The rate of attack of an alkaline etching bath on aluminum is dependent to some degree on such factors as the concentration of the alkali and the temperature at which the bath is operated. The metal additives in no way counteract the increase in rate obtained by increasing alkali concentration and temperature and the additives may, if desired, be used in conjunction with higher alkali concentrations and temperatures to obtain even faster etching rates.

Referring more specifically to the accelerants employed in the practice of the present invention, molybdenum, only in the hexavalent form, is effective in increasing etch- 2,975,039 Patented Mar. 14, 1961 ing rates. Nickel is effective only in the divalent form, and iron is effective in either the dior trivalent form.

Apparently, the metal ions may be added to the etching solution in the form of any salt that is wholly or partially soluble in water. Thus a completely waterinsoluble iron compound such as hematite is apparently ineffective. ion, which form insoluble hydroxides on addition to alkaline solutions, might not be expected to produce an effect of any kind in an alkaline etching bath, but surprisingly, provided the ions are introduced into the bath in the form of wholly or partially water soluble salts, they produce an accelerating effect. Although the exact mechanism is not known, it may be that a minute but suificient amount of the ion is present in the form of a colloid or complex of some description to provide the desired effect. For example, a soluble ferric salt, upon addition to a hot alkaline bath, will cause some turbidity, or even form a perceptible precipitate, presumably of highly insoluble ferric hydroxide, but it still acts effectively to increase the etching rate of the bath. It follows from this unexpected characteristic of the accelerating metal ions, i.e., that they are effective even when not completely in solution in the alkali, that these metal ions, in their effective valence states, may be added to the'baths as any one of their water-soluble salts. The soluble inorganic salts, such as the chlorides, bromides, sulfates or nitrates or the organic salts such as the formates or acetates are examples of the forms in which these ions may be used. The hexavalent form of molybdenum may,

for example, be supplied as molybdic acid or as sodium.

molybdate.

Most aluminum etching baths are made up using caustic soda, for reasons of economy, although other etching alkalies can be used. Similarly, although caustic soda is the preferred alkali for the practice of the present invention, other alkalies such as potassium hydroxide, lithium hydroxide or trisodium phosphate may be used. In conventional practice, the caustic soda concentration of aluminum etching baths is generally in the range of from 1% to not more than about 20%, most chem milling being done within the range of 8 to 15% since the lower concentrations generally do not contain sufficient caustic to remove the large amount of metal required and at the and applicants co-pending application Serial No. 499,502,

filed March 31, 1955, now US. Patent No. 2,872,301, the preferred scale inhibitors generally being the glucamines, sorbitol and gluconate. Where such scale inhibitors are used they are generally present in amounts of l to 5%, based on the amount of alkali. .It ispreferred to include one or more antiscalants in the-fast etching compositions and process of the present invention, although such antiscalant is not necessary for successful acceleration but is included primarily to prevent scale formation. When large amounts of aluminum are to be.

removed, as is generally the case where very fast etching baths are employed, it is advantageous to prevent the copious scale formation that would ordinarily result from the dissolution of these large amounts. This may be Metal ions such as the ferric or ferrous TABLE r Percent Etching Rates 1 Accel- Accelerant erant Aluminum Alloy 2 based on caustic soda 3 28 38 528 24S None .67 70 .70 75 Ferric ion (as FeCla) 86 88 .93 .75 81 Ferrous ion (as F6804) 48 .85 .87 .81 Molybdenum ion (as NazMoOom- 0 35 .80 77 80 Nickelous ion (as NiSO4.7H20)-.. 0 .83 76 .81

1 Figures represent grams aluminum dissolved or 3 minutes per 18.5 sq. in. of surface, in a 4% caustic bath containing 1 0 sorbitol based on the caustic; 150 F.

2 For compositions of the designated alloys, see for example, the Handbook of Chemistry and Physics, Chemical Rubber Publishing Company, 1948, pp. 1266 and 1276.

3 Figures represent weight percent of metal ion present, and not of metal salt.

Very small concentrations of the metal additives? on the order of 0.2 to 2.5% metal ion based on the weight of alkali, will provide increased acceleration for alkali etching baths. Addition of quantities in excess of 2.5%, though having no apparent harmful effect does not appear to accelerate the aluminum dissolution further. The preferred range of concentration for the accelerating metal ion is generally in the order of from 0.5 to 1% based on the weight of the alkali, the 1% upper limit being dictated primarily by the cost of the metal salt used.

In the practice of the present invention, an etching bath containing 1 to 20% of an etching alkali is made up with a salt of one of the metals of the group consist ing of iron, nickel and molybdenum of the valances heretofore mentioned, in an amount sufficient to provide 0.2 to 2.5% metal ion based on the caustic, and, if desired, 1 to 5% based on the alkali, of an antiscalant. The components of the bath may be mixed first, as for example in a dry mix that can be packaged and stored, which is later added to water in preparing the etching bath separately. The etching bath maybe operated at any suitable temperature such as 120 to 212 F., and replenished with the necessary components when it becomes depleted through use.

In the preferred practice, where a very rapid etch is desired without going to the high alkali concentrations of co-pending application Serial No. 544,584, hereinabove identified, a relatively strongly alkaline etching bath on the order of 8 to caustic soda, is employed at relatively high temperatures, of about 180 to 200 F. The accelerating metal is added in amounts of from 0.5 to 1% based on the weight of the alkali. From 1 to 5% of an antiscalant based on the alkali, may also be added, the preferred antiscalant being any of the group consisting of sorbitol, gluconate and the glucarnines. When the aluminum stock has been etched in this bath to the desired degree, it is removed, rinsed and then preferably desmutted in a 15 to nitric acid solution.

It is generally preferred to provide all the etching ingredients in a single package in a dry form, the etching solution then being prepared by adding this to water. In preparing such a composition, etching alkali, preferably caustic soda, is mixed with the accelerant metal ion in amounts such that the final composition will contain about 0.2 to 2.5 parts by weight metal ion per 100 parts of caustic. In the preferred practice, suflicient scale inhibitor is included to give a composition containing about 0.2 to 2.5 parts by weight of metal ion and 1 to 5 parts scale inhibitor per parts of caustic.

An example of such a dry composition may contain, for each 100 parts of caustic soda, about 0.6 to 7.3 parts by weight ferric chloride (0.2 to 2.5 parts ferric ion) and 1 to 5 parts by weight sorbitol. In preparing the etching bath the etching composition is then added to water to give a solution having the caustic concentration desired, which is generally in the order of 1 to 20%.

Though ferric chloride was used in the above illustration, as previously indicated any of the metal ions of the group consisting of hexavalent molybdenum, divalent nickel or iron or trivalent iron also serve to substantially increase the etching rate. Thus in place of the ferric chloride one could use any of the salts of these metals for example, nickel bromide, nickel sulfate, nickel chloride, ferric citrate, ferric sulfate, ferrous acetate, ferrous carbonate, ferrous sulfate or molybdic acid or so dium molybdate. The invention, however, is not limited to these compounds alone since apparently the acceleration is caused by the presence of any of the above indicated metal ions, and other compounds which contain these metal ions and which are partially or wholly soluble in Water can be used. Such compounds are readily recognized by a chemist.

In preparing the dry formulations just described it is sometimes advantageous to add certain other ingredients such as pine oil or nitrates in small quantities, as dedusting or brightening agents, etc.

In order to illustrate more clearly the benefit of the results obtained by the invention, the following examples are given:

Example I The rate of dissolution of 38 aluminum in a 4% caustic soda solution was found to be 0.0115 gram per minute per square inch of surface area, the solution being held at F. By comparison, an exactly similar solution, with the exception that it contained 0.4 gram of ferric chloride per liter solution, produced a dissolution rate of 0.0150 gram per minute per square inch of surface, an increase of 30%.

The significance of this increase becomes more apparent if it is considered that, whereas it would require about six and one-half hours to dissolve a one pound article of aluminum having a surface area of 100 square inches in an unaccelerated bath, the same article could be dissolved in only about five hours by the addition of the ferric ion, all other factors being equal. Less obvious than the time saved are the savings in labor costs and fuel requirements for elevated bath temperatures. These are considerable in large scale etching operations.

Example II An etching bath similar to the accelerated solution of Example I was used to etch articles of 35 aluminum for an extended period of time. The bath was replenished from time to time with caustic soda and ferric chloride in the same proportions. It was found, however, that on continued use hard scale formed on the surfaces of the tank exposed to the solution. A fresh bath was prepared, in which was included 2% sorbitol based on the weight of the caustic soda, and in which the caustic and ferric ion were present in the same amounts as before. By this means, the formation of hard scale was prevented, while the accelerating effect of the ferric ion was undiminished.

After-treatment of the etched articles consisted in rinsing with water and desmutting by immersion in aqueous 15% nitric acid solution for a few seconds. The etched articles were uniformly smooth and clean on completion of the process. Addition of the antiscalant did not change the final appearance of the etched articles.

Having described my invention, I claim:

1. The method of etching aluminum including aluminum base alloys involving relatively rapid solution of substan- Lin" tial quantities of said aluminum in a hot aqueous solution of an etching alkali, said method comprising treating the surface of said aluminum with an aqueous solution containing from 1 to 20% by Weight of an etching alkali and containing at least one metal ion of the group consisting of hexavalent molybdenum, divalent nickel, divalent iron and trivalent iron at a temperature of from 120 to 212 F., said metal ion being present in an amount of at least 0.2% by weight based on the weight of said alkali.

2. The method of claim 1 wherein said solution also contains an antiscal-ant.

3. The method of claim 1 wherein said etching alkali is primarily caustic soda.

4. The method of etching aluminum including aluminum base alloys involving relatively rapid solution of substantial quantities of said aluminum in a hot aqueous solution of an etching alkali, said method comprising treating the surface of said aluminum with an aqueous solution of caustic soda having a temperature of 180 to 200 F.,

based on the caustic soda of at least one antiscalant of the group consisting of sorbitol, glucamine and gluconate, and 0.5 to 1% based on the caustic soda of a metal of the group consisting of hexavalent molybdenum, divalent 5 nickel, divalent iron and trivalent iron.

References Cited in the tile of this patent UNITED STATES PATENTS OTHER REFERENCES Thorpes Dictionary of Applied Chemistry, 4th ed., 1950, vol. X TP 9T7 1937 C2, pubLby Longmans, New York city. (Copy in Div. 59.) P. 855, col. 2, par. 2 under said solution containing 8 to 15% caustic soda, 1 to 5% 20 Chem. Prop. of NaOH. 

1. THE METHOD OF ETCHING ALUMINUM INCLUDING ALUMINUM BASE ALLOYS INVOLVING RELATIVELY RAPIED SOLUTION OF SUBSTANTIAL QUANTITIES OF SAID ALUMINUM IN A HOT AQUEOUS SOLUTION OF AN ETCHING ALKALI, SAID METHOD COMPRISING TREATING THE SURFACE OF SAID ALUMINUM WITH AN AQUEOUS SOLUTION CONTAINING FROM 1 TO 20% BY WEIGHT OF ANS ETCHING ALKALI AND CONTAINING AT LEAST ONE METAL ION OF THE GROUP CONSISTING OF HEXAVALENT MOLYBDENUM, DIVALENT NICKEL, DIVALENT IRON AND TRIVALENT IRON AT A TEMPERATURE OF FROM 120* TO 212* F., SAID METAL ION BEING PRESENT IN AN AMOUNT OF AT LEAST 0.2% BY WEIGHT BASED ON THE WEIGHT OF SAID ALKALI. 